In the processing industries many substances in liquid form are used in manufacturing processes to fabricate finished products. In the production of integrated circuits and the like, the liquids used often have chemical properties that render them extremely difficult to handle, including high toxicity, high corrosivity, high vapor pressure, and high flammability. In addition, it is often absolutely necessary to maintain extreme purity of the chemicals. Otherwise, impurities introduced into the integrated circuit wafers will render the devices inoperative or defective.
Generally speaking, the process chemicals used in semiconductor and integrated circuit manufacturing are stored and used in containers or tanks formed of corrosion resistant materials that do not introduce impurities into the chemicals, such as glass, quartz, and occasionally stainless steel. The tanks are sealed from contamination from the ambient air, and the liquids are transferred through closed pumping and pressure systems using inert gases. Thus the amounts of liquids in the containers are generally not directly observable. Furthermore, the processing machines for integrated circuits and the like have reached a level of automation which requires constant monitoring of the amounts of process chemicals available to the machines and in use within the machines.
Also, it is important that the amount of process chemical being stored in tanks is well known and controlled, to prevent overflow and spillage of these dangerous substances, to assure delivery of the proper amount of the substances to the fabrication apparatus when required, and to prevent unsafe storage conditions. For example, some process materials will explode spontaneously if the liquid level within the container falls below a minimum amount.
The task of sensing liquid levels within a container or tank or process chamber without contaminating the liquid is problematic at best. Any probe placed within the liquid must be cleaned more thoroughly than a surgical instrument, and this restriction is not consonant with production floor manufacturing. In the prior art, techniques such as weigh scales to check the net mass of liquid retained in a container have been used, but this is an expensive approach, both in equipment investment and in labor devoted to monitoring and maintaining the scales. Other techniques include the use of an inert gas such as nitrogen bubbled through the liquid, with a detector to sense the change in fluid resistance in the gas when the bubbling orifice extends above the liquid surface. Capacitive detectors have also been used, generally involving a conductive probe disposed within the container, and a conductive electrode secured about the container. Capacitive sensors are prone to false readings, due for example to large masses being moved about the area of the container being monitored. Both these techniques involve placing a foreign object in the container, with the attendant problems of contamination. Clearly there is a need in the art for a non-invasive sensing system for detecting liquid level within a container; that is, a system that is relatively inexpensive, reliable, and accurate.